Although immigration has slowed, higher birth rates among Hispanics make them the fastest growing group. Births, rather than immigration, accounted for about two-thirds of the 1.47 million increase in the Hispanic population in 2008, according to KennethJohnson, demographer at the University of New Hampshire's Carsey Institute. In addition, Hispanics are younger, on average, than the overall population. Births among Hispanics outpaced deaths by nearly 10 to one.

Forty-seven percent of children under 5 are minorities, as are 43% of young people under age 20.

"It's a cumulative effect of immigration," says Jeffrey Passel of the Pew Hispanic Center. "We've built up a population of Hispanics, and increasingly they're native born."

As the median age among non-Hispanic whites increases — it's 41.1 compared with 27.7 for Hispanics — so will the racial and ethnic generation gap, demographers say.

"A lot of these Boomers are going to be relying on this younger generation to take care of them in a lot of ways," says Mark Mather of the Population Reference Bureau. "In another generation, this is going to be our workforce that is supporting Social Security."

Orange County, Fla., home of Walt Disney World, is one of six U.S. counties where the population became majority-minority in 2008: more than half the population are in groups other than non-Hispanic whites.

That's "not a surprise" to Orange County Mayor Richard Crotty, who says the county has always been "a snapshot of what America looks like." Nearly 10% of the nation's 3,142 counties have a minority population above 50%.

The demographic shift is most dramatic among "kids under 20," Mather says. "They really are the groups that are driving these changes."

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